A closed system for an adaptation of a level of a vehicle is known from DE 101 60 972 C1 corresponding to U.S. 2003 0101791 A1 and U.S. Pat. No. 6,845,988 B2. In such system a quantity of air is moved due to a pressure difference or due to the activity of a pump from an air container to a bellow or in opposite direction in order to change the level of a vehicle. Prior to such change of the level of the vehicle the temperature of the environment of the vehicle is measured. The actual mass of the quantity of air inside the closed system is calculated from the air pressure in the different components of the air suspension system, an actual level of the vehicle and the measured temperature of the environment. Furthermore, a desired mass of the air and a desired pressure of the air inside the closed system are calculated from the level of the vehicle, the temperature of the environment and the maximum change of the level of the vehicle which is possible from the actual level of the vehicle. In case of the actual pressure being smaller than the desired pressure an augmentation of the actual mass of air in the system is necessary. However, in case of the actual pressure exceeding the desired pressure the actual mass of air is decreased. Such changes result in adapting the pressure and the mass of air inside the air suspension system also in case of changes of the temperature of the environment such that the pressure and the mass of air is always sufficient to cause the maximum change of the level of the vehicle. A change of the mass and the pressure of the air inside the system might be provided by a pneumatic exchange of the air suspension system which could be done by means of an increase of the mass of air by a pump or by draining air to the environment. In summary, such technique relies on the assumption that the pneumatic conditions of the air suspension system should be held constant or comparable also in case of changes of the temperature in order to achieve a constant behavior throughout a change of a level of a vehicle.
Another method is known from EP 1 375 207 A2. According to that method for a change of the balance of the load of different axles the level of at least one axle is changed. The method aims at providing a minimum load for a steered front axle. By means of a change of the level of a rear axle the load of the steered front axle might be influenced. The load of the front axle is measured throughout a change of the level of the rear axle. In the following it is observed if the measured load of the steered front axle drops below a predetermined minimum load. In case of such criterion not being fulfilled the change of the level of the rear axle is continued until the minimum load is reached. Such method relies on a control under feedback of an actual value of a load of an axle.
Also document DE 43 33 823 A1 relates to an adaptation of a heavy duty vehicle from an actual level to a desired level. In order to provide for a fast adaptation process control valves with large opening cross-sections are used leading to a decreased precision of the approximation of the desired level. On the other hand, it is mentioned that the air inside the air suspension system comprises elastic features so that any actuating of the related valves into a closed state do not immediately finish any changes of the level but require further adaptation steps. In order to avoid such problems DE 43 33 823 A1 summarizes known methods basing on a control of the air suspension system to reach an intermediate level located between the starting level and the desired level. If such intermediate level is reached the valves are opened and closed in several steps in an intermittent way in order to approach the precise desired level. According to an alternative embodiment a valve responsible for the change of the level is not closed at the moment that the desired level is reached. However, throughout the change of the level a hold-back time is calculated from a measured speed of the change of the level. Throughout the change of the level of the vehicle the actual level is measured and compared to the desired level decreased by the hold-back level. In case of the actual level exceeding the desired level decreased by the hold-back level the valve is actuated in a closed state under the assumption that following to closing the valve any subsequent changes of the level are covered by the hold-back level. In case of further measurements showing that such assumption is not valid, the hold-back level is adjusted for future adaptations of the level of the vehicle. In special cases, the difference between the starting level and the desired level could be below a critical value. This might result in the time interval for a change of the level of the vehicle being too short to measure the speed of the change of the level. Only in such cases the opening interval is determined from an a priori known relationship between the difference of the starting level and the desired level and response time of the used valve. According to DE 43 33 823 A1 different load conditions of the heavy duty vehicle might be considered by making the aforementioned critical value of the difference between the starting level and the desired level dependent on the loading condition of the heavy duty vehicle.
Furthermore, DE 196 40 149 A1 discloses a control of the actual value of the level of a vehicle throughout the changes of the level. By means of an adapted linear relationship the after-running after the step of closing the valve is stored in dependence on a measured velocity of the change of the level of the vehicle. The after-running level is closely related to the hold-back level according to DE 43 33 823 A1. In case of the actual level of the vehicle exceeding the desired level of the vehicle decreased by the after-running level the valve being responsible for the change of the level is actuated in a closed state.